1. Field of the Invention
This invention relates to a DMOS (Double-diffused MOS) transistor.
2. Description of the Related Art
The DMOS transistor is a MOS field effect transistor in which a source layer and a body layer that makes a channel are formed by double diffusion of impurities, and is used as a power semiconductor device for a power supply circuit, a driver circuit or the like.
In recent years, because of requirements for electronic equipment such as mobile phones to reduce their size and power consumption, the DMOS transistor is required to reduce its ON resistance (a resistance of the DMOS transistor when it is turned on).
Technologies to reduce the ON resistance and to improve a withstand voltage and the like of the DMOS transistor are disclosed in Japanese Patent Application Publication Nos. H10-233508, 2004-39773, 2004-79800, 2007-128978, 2007-535813 and 2008-34737.
Forming a silicide layer, which is a product of reaction between silicon and a transition metal such as cobalt or titanium, on a surface of a source layer or the like is effective to reduce the ON resistance of the DMOS transistor. However, there is a problem that the silicide layer tends to cause convergence of an electric field and an electric current in the transistor and deteriorate strength of the DMOS transistor against electrostatic discharge, since the silicide layer is low in resistance.